Oil is often produced as part of an emulsion of water and oil. An oil water separator may be used to separate the oil from the water. In one embodiment such a separator comprises an elongated vessel, through which the oil-water mixture flows horizontally to a height-controllable weir on one side of the vessel. During the time interval in which a volume element of the oil water mixture flows through the vessel, droplets in the emulsion coalesce. Various measures may be applied in the vessel to promote removal of the emulsion, such as heating, application of electrostatic fields, adding agents to reduce oil-water surface tension etc. Conventionally, a predetermined flow speed is selected so that there is always sufficient time for substantially all droplets to coalesce in the time interval in which a volume element of the oil water mixture flows through the vessel. As a result a water body forms in the vessel at the bottom and an oil body forms at the top, possibly with a gas phase above the oil. The height of the weir is adjusted according to the height of the separation between the bodies of water and oil, in order to collect an oil fraction free of water and/or waste water that is substantially free of oil.
In order to be able to adjust the weir height to the height of the separation between oil and water, a measuring system is used to measure the height of this separation. Various solutions have been proposed for this purpose.
From an article by Bukhari et al, published in Sensors 6 (2006) pages 380-389 and titled “Multi-interface Level Sensors and New Development in Monitoring and Control of Oil Separators” it is known to measure oil water separation levels by using a series of ultrasonic transmitter-receiver pairs at different heights along vertical walls of a separator vessel. Each pair can be used to determine the speed of sound or sound absorption at a different level, from which it can be determined whether oil or water is present at that level. It is also known to provide such transmitter-receiver pairs at different heights on a probe that is inserted in the separator vessel. This makes it possible to overcome the problem that measurements at individual heights with ultrasound transducers on the walls of a vessel without vertical walls are not possible (because ultrasound usually radiates only in a direction normal to the wall). However, transmitter-receiver pairs at many different heights may be needed to obtain sufficient height resolution.
Bukhari et al also describe a method that works with a single receiver-transmitter pair on the wall of a vessel with rounded cross-section. In this method a transmitter and receiver are located at the bottom of the vessel, to transmit and receive back ultrasound through the emulsion above the bottom. From the time delay of received reflections from interfaces between different phases in the vessel the levels of these interfaces can be estimated. However, the oil water interface produces only a weak reflection, which is hard to detect. When an emulsion is present between the oil and the water, it is even more difficult to obtain useful measurements.
Form an article by Jaworski et al, published in the Journal of Petroleum Science and Engineering 68 (2009) pages 47-59, and titled “On-Line measurement of separation dynamics in primary gas/oil/water separators: Challenges and technical solutions—A review”, it is known to use tomography for monitoring heterogeneous mixtures in separators. The article describes an example of electrical capacitance tomography, using capacitor plates on the periphery of the separator and measurement of capacitance between respective pairs of plates. The article also describes the possibility of using gamma-ray tomography or combinations of multiple modes of tomography. As noted by the article appropriate reconstruction algorithms may be used to obtain a cross-sectional image of the distribution of the measured property such as the dielectric constant or resistivity, but no specific algorithm is described. Conventionally for gamma rays, tomography involves back-projection, identifying paths between each pair of electrodes, and assigning contributions to the fluid properties at positions along the path in proportion to the measured capacitance between the electrodes.
Compared to level sensing tomography has the advantage that an image of a measurable property such as electrical polarizability or gamma ray absorption as a function of position can be obtained without requiring transmitters and receivers at equal height that “see” each other. When the expected value of the measurable property in the tomographic image is different for water and oil, the tomographic image can be used to control the height of the weir, by setting it to the height in the image where the value of the measurable property substantially reaches the expected value for oil.
Ultrasound tomography is known per se, and it could be applied to a separator vessel, by providing for ultrasound transducers. The speed of sound in oil and water is different, so that measurement of ultrasound travel times can be used to distinguish oil and water bodies. When ultrasound tomography is used, there is no need to provide equal height pairs of ultrasound transmitters and receivers at a large number of heights along the vessel.
However, it has been found that sometimes the results obtained with ultrasound tomography in a separator vessel still may still be inaccurate. It has been found that this occurs especially when there is a substantial height of emulsion left at the measurement stage. When equal height pairs of transducers are used, this may have the effect that the pairs in the emulsion range produce results that cannot clearly attributed to either water or oil. For tomography with ray paths at an angle to the horizontal the result is worse, as it also makes the determination of the limits of the water and oil bodies inaccurate. Of course these problems are normally avoided by performing the ultrasound measurements near the weir and using a flow speed that ensures sufficient time for removing the emulsion.
Determination of the droplet size distribution in an emulsion from the frequency dependence of ultrasound attenuation is known from an article by F. Alba et al., titled “Acoustic spectroscopy as a technique for the particle sizing of high concentration colloids, emulsions and suspension”. Alba et al. show that the size distribution of droplets in an oil-water emulsion can be determined by fitting the parameters of a model to measured frequency dependent ultrasound attenuation, using a model that predicts the frequency dependent attenuation as a function of parameters of the droplet size distribution.
In principle, Alba et al. make it possible to measure the cumulative droplet size distribution in a vertical column with an oil-water mixture, by measuring ultrasound wavelength dependent attenuation along a vertical ray path. But in oil water separator vessel, wherein gas is present above the mixture, reflections from the gas-liquid interface make such measurements. Alba et al. do not consider determination of position dependent droplet size distribution. Sensing horizontal transmission at discrete heights would require many transmitter reducer pairs, and is impossible altogether when the vessel has a rounded wall and ultrasound propagates normal to the wall. Alba et al. do not consider effects of inhomogeneity on the part of the emulsion that is traversed by ultrasound in the case of rays at a non-zero angle to the variation of the distribution, or application of determination of droplet size distribution to control of an oil-water separator.